/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.apache.activemq.util;

public class JenkinsHash {

    private static final long INT_MASK = 0x00000000ffffffffL;
    private static final long BYTE_MASK = 0x00000000000000ffL;

    private static final JenkinsHash _instance = new JenkinsHash();

    public static JenkinsHash getInstance() {
        return _instance;
    }

    private static long rot(long val, int pos) {
        return ((Integer.rotateLeft((int) (val & INT_MASK), pos)) & INT_MASK);
    }

    /**
     * Calculate a hash using all bytes from the input argument, and
     * a seed of -1.
     * @param bytes input bytes
     * @return hash value
     */
    public int hash(byte[] bytes) {
        return hash(bytes, bytes.length, -1);
    }

    /**
     * Calculate a hash using all bytes from the input argument, and
     * a seed of -1.
     * @param bytes input bytes
     * @return hash value
     */
    public int hash(byte[] bytes, int initVal) {
        return hash(bytes, bytes.length, initVal);
    }

    /**
     * taken from  hashlittle() -- hash a variable-length key into a 32-bit value
     *
     * @param key     the key (the unaligned variable-length array of bytes)
     * @param nbytes  number of bytes to include in hash
     * @param initval can be any integer value
     * @return a 32-bit value.  Every bit of the key affects every bit of the
     * return value.  Two keys differing by one or two bits will have totally
     * different hash values.
     * <p>
     * <p>The best hash table sizes are powers of 2.  There is no need to do mod
     * a prime (mod is sooo slow!).  If you need less than 32 bits, use a bitmask.
     * For example, if you need only 10 bits, do
     * <code>h = (h & hashmask(10));</code>
     * In which case, the hash table should have hashsize(10) elements.
     * <p>
     * <p>If you are hashing n strings byte[][] k, do it like this:
     * for (int i = 0, h = 0; i < n; ++i) h = hash( k[i], h);
     * <p>
     * <p>By Bob Jenkins, 2006.  bob_jenkins@burtleburtle.net.  You may use this
     * code any way you wish, private, educational, or commercial.  It's free.
     * <p>
     * <p>Use for hash table lookup, or anything where one collision in 2^^32 is
     * acceptable.  Do NOT use for cryptographic purposes.
     */
    public int hash(byte[] key, int nbytes, int initval) {
        int length = nbytes;
        long a, b, c;       // We use longs because we don't have unsigned ints
        a = b = c = (0x00000000deadbeefL + length + initval) & INT_MASK;
        int offset = 0;
        for (; length > 12; offset += 12, length -= 12) {
            a = (a + (key[offset + 0] & BYTE_MASK)) & INT_MASK;
            a = (a + (((key[offset + 1] & BYTE_MASK) << 8) & INT_MASK)) & INT_MASK;
            a = (a + (((key[offset + 2] & BYTE_MASK) << 16) & INT_MASK)) & INT_MASK;
            a = (a + (((key[offset + 3] & BYTE_MASK) << 24) & INT_MASK)) & INT_MASK;
            b = (b + (key[offset + 4] & BYTE_MASK)) & INT_MASK;
            b = (b + (((key[offset + 5] & BYTE_MASK) << 8) & INT_MASK)) & INT_MASK;
            b = (b + (((key[offset + 6] & BYTE_MASK) << 16) & INT_MASK)) & INT_MASK;
            b = (b + (((key[offset + 7] & BYTE_MASK) << 24) & INT_MASK)) & INT_MASK;
            c = (c + (key[offset + 8] & BYTE_MASK)) & INT_MASK;
            c = (c + (((key[offset + 9] & BYTE_MASK) << 8) & INT_MASK)) & INT_MASK;
            c = (c + (((key[offset + 10] & BYTE_MASK) << 16) & INT_MASK)) & INT_MASK;
            c = (c + (((key[offset + 11] & BYTE_MASK) << 24) & INT_MASK)) & INT_MASK;

            /*
             * mix -- mix 3 32-bit values reversibly.
             * This is reversible, so any information in (a,b,c) before mix() is
             * still in (a,b,c) after mix().
             *
             * If four pairs of (a,b,c) inputs are run through mix(), or through
             * mix() in reverse, there are at least 32 bits of the output that
             * are sometimes the same for one pair and different for another pair.
             *
             * This was tested for:
             * - pairs that differed by one bit, by two bits, in any combination
             *   of top bits of (a,b,c), or in any combination of bottom bits of
             *   (a,b,c).
             * - "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
             *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
             *    is commonly produced by subtraction) look like a single 1-bit
             *    difference.
             * - the base values were pseudorandom, all zero but one bit set, or
             *   all zero plus a counter that starts at zero.
             *
             * Some k values for my "a-=c; a^=rot(c,k); c+=b;" arrangement that
             * satisfy this are
             *     4  6  8 16 19  4
             *     9 15  3 18 27 15
             *    14  9  3  7 17  3
             * Well, "9 15 3 18 27 15" didn't quite get 32 bits diffing for
             * "differ" defined as + with a one-bit base and a two-bit delta.  I
             * used http://burtleburtle.net/bob/hash/avalanche.html to choose
             * the operations, constants, and arrangements of the variables.
             *
             * This does not achieve avalanche.  There are input bits of (a,b,c)
             * that fail to affect some output bits of (a,b,c), especially of a.
             * The most thoroughly mixed value is c, but it doesn't really even
             * achieve avalanche in c.
             *
             * This allows some parallelism.  Read-after-writes are good at doubling
             * the number of bits affected, so the goal of mixing pulls in the
             * opposite direction as the goal of parallelism.  I did what I could.
             * Rotates seem to cost as much as shifts on every machine I could lay
             * my hands on, and rotates are much kinder to the top and bottom bits,
             * so I used rotates.
             *
             * #define mix(a,b,c) \
             * { \
             *   a -= c;  a ^= rot(c, 4);  c += b; \
             *   b -= a;  b ^= rot(a, 6);  a += c; \
             *   c -= b;  c ^= rot(b, 8);  b += a; \
             *   a -= c;  a ^= rot(c,16);  c += b; \
             *   b -= a;  b ^= rot(a,19);  a += c; \
             *   c -= b;  c ^= rot(b, 4);  b += a; \
             * }
             *
             * mix(a,b,c);
             */
            a = (a - c) & INT_MASK;
            a ^= rot(c, 4);
            c = (c + b) & INT_MASK;
            b = (b - a) & INT_MASK;
            b ^= rot(a, 6);
            a = (a + c) & INT_MASK;
            c = (c - b) & INT_MASK;
            c ^= rot(b, 8);
            b = (b + a) & INT_MASK;
            a = (a - c) & INT_MASK;
            a ^= rot(c, 16);
            c = (c + b) & INT_MASK;
            b = (b - a) & INT_MASK;
            b ^= rot(a, 19);
            a = (a + c) & INT_MASK;
            c = (c - b) & INT_MASK;
            c ^= rot(b, 4);
            b = (b + a) & INT_MASK;
        }

        //-------------------------------- last block: affect all 32 bits of (c)
        switch (length) {                   // all the case statements fall through
            case 12:
                c = (c + (((key[offset + 11] & BYTE_MASK) << 24) & INT_MASK)) & INT_MASK;
            case 11:
                c = (c + (((key[offset + 10] & BYTE_MASK) << 16) & INT_MASK)) & INT_MASK;
            case 10:
                c = (c + (((key[offset + 9] & BYTE_MASK) << 8) & INT_MASK)) & INT_MASK;
            case 9:
                c = (c + (key[offset + 8] & BYTE_MASK)) & INT_MASK;
            case 8:
                b = (b + (((key[offset + 7] & BYTE_MASK) << 24) & INT_MASK)) & INT_MASK;
            case 7:
                b = (b + (((key[offset + 6] & BYTE_MASK) << 16) & INT_MASK)) & INT_MASK;
            case 6:
                b = (b + (((key[offset + 5] & BYTE_MASK) << 8) & INT_MASK)) & INT_MASK;
            case 5:
                b = (b + (key[offset + 4] & BYTE_MASK)) & INT_MASK;
            case 4:
                a = (a + (((key[offset + 3] & BYTE_MASK) << 24) & INT_MASK)) & INT_MASK;
            case 3:
                a = (a + (((key[offset + 2] & BYTE_MASK) << 16) & INT_MASK)) & INT_MASK;
            case 2:
                a = (a + (((key[offset + 1] & BYTE_MASK) << 8) & INT_MASK)) & INT_MASK;
            case 1:
                a = (a + (key[offset + 0] & BYTE_MASK)) & INT_MASK;
                break;
            case 0:
                return (int) (c & INT_MASK);
        }

        /*
         * final -- final mixing of 3 32-bit values (a,b,c) into c
         *
         * Pairs of (a,b,c) values differing in only a few bits will usually
         * produce values of c that look totally different.  This was tested for
         * - pairs that differed by one bit, by two bits, in any combination
         *   of top bits of (a,b,c), or in any combination of bottom bits of
         *   (a,b,c).
         *
         * - "differ" is defined as +, -, ^, or ~^.  For + and -, I transformed
         *   the output delta to a Gray code (a^(a>>1)) so a string of 1's (as
         *   is commonly produced by subtraction) look like a single 1-bit
         *   difference.
         *
         * - the base values were pseudorandom, all zero but one bit set, or
         *   all zero plus a counter that starts at zero.
         *
         * These constants passed:
         *   14 11 25 16 4 14 24
         *   12 14 25 16 4 14 24
         * and these came close:
         *    4  8 15 26 3 22 24
         *   10  8 15 26 3 22 24
         *   11  8 15 26 3 22 24
         *
         * #define final(a,b,c) \
         * {
         *   c ^= b; c -= rot(b,14); \
         *   a ^= c; a -= rot(c,11); \
         *   b ^= a; b -= rot(a,25); \
         *   c ^= b; c -= rot(b,16); \
         *   a ^= c; a -= rot(c,4);  \
         *   b ^= a; b -= rot(a,14); \
         *   c ^= b; c -= rot(b,24); \
         * }
         *
         */
        c ^= b;
        c = (c - rot(b, 14)) & INT_MASK;
        a ^= c;
        a = (a - rot(c, 11)) & INT_MASK;
        b ^= a;
        b = (b - rot(a, 25)) & INT_MASK;
        c ^= b;
        c = (c - rot(b, 16)) & INT_MASK;
        a ^= c;
        a = (a - rot(c, 4)) & INT_MASK;
        b ^= a;
        b = (b - rot(a, 14)) & INT_MASK;
        c ^= b;
        c = (c - rot(b, 24)) & INT_MASK;

        return (int) (c & INT_MASK);
    }

}
